1. Field of the Invention
This invention relates to a process for producing anhydrous hydrogen fluoride and finely divided silica from waste fluosilicic acid solutions.
2. Brief Description of the Prior Art
In the industrial manufacture of phosphoric acid, phosphate rock is treated with sulfuric acid, thereby resulting in a byproduct of aqueous fluosilicic acid which is produced in large quantities and is normally discarded. Because of the attendant pollution problems associated with discarding this waste product and the potential value of the waste product as a source of silica and hydrogen fluoride, an economical and efficient process is highly desired for recovery of these important and useful industrial materials from by-product fluosilicic acid.
Finely divided silica has been historically used as a white pigment in paints for decorating and protecting the surfaces of a wide variety of articles and products such as automobiles, household interiors and exteriors.
Anhydrous hydrogen fluoride is widely used in applications such as polishing, etching and frosting of glass; for pickling copper, brass, stainless and other alloy steels; electro-polishing of metals; manufacture of fluorocarbons; manufacture of cryolite; cleaning stone and brick; purification of filter paper and graphite, and dissolving ores and cleaning castings.
U.S. Pat. No. 3,787,304 (Chlanda, Liu, 1974) describes a process wherein aqueous fluosilicic acid is converted to a fluoride salt solution and silica by treatment with hydroxide base solution. The silica is filtered off and the fluoride salt solution is then converted to an aqueous solution of hydrogen fluoride by an electrodialytic water-splitting process. The base solution is recycled, and the aqueous acid solution is then treated by membrane electrodialysis to produce a concentrated aqueous solution of hydrogen fluoride, from which anhydrous hydrogen fluoride is subsequently obtained by distillation. However, no suggestion is made regarding the utility of the process in treating ternary liquid systems, such as hydrogen fluoride/fluosilicic acid/water.
U.S. Pat. No. 3,969,485 (1976) describes a process for converting fluosilicic acid to silica and hydrogen fluoride by treating fluosilicic acid with sulfuric acid to produce hydrogen fluoride and silicon tetrafluoride which is treated with water in a vapor-phase process to form silica. However, the hydrogen fluoride is recovered in the process by treating aqueous solutions of hydrogen fluoride with sulfuric acid to eliminate water, followed by distillation.
A process is described in U.S. Pat. No. 4,036,938, (1977) in which an aqueous solution of fluosilicic acid is pyrolyzed to form silica and a dilute aqueous solution of hydrogen fluoride. The aqueous solution of hydrogen fluoride is subsequently treated with sulfuric acid to remove water and the hydrogen fluoride is isolated in the process by distillation.
A related process is described in U.S. Pat. No. 4,008,130 (1977) for producing the ternary azeotropic system of HF/H.sub.2 SiF.sub.6 /H.sub.2 O in various processes, to avoid the undesirable formation of SiO.sub.2 during use. However, the teaching of the reference is not directed to the recovery of HF from the azeotropic system.
A process is also described in Russian Patent USSR No. 174,610 (1965) involving an electrodialysis method of concentrating substances, including fluosilicic acid, wherein sulfuric acid is utilized in the anolyte.
A process for recovery of hydrofluoric acid is described in U.S. At. Energy Comm. ORNL-208, 14 pp. (1956) in which a by-product aqueous solution of hydrofluoric acid from the hydrofluorination of uranyl dioxide is subjected to electrodialysis followed by distillation to recover hydrofluoric acid. However, the authors believe that the process is not economical for commercial use.
What is needed and what the prior art does not provide is a process for manufacturing silica and anhydrous hydrogen fluoride from waste aqueous fluosilicic acid solution without the necessity of using large amounts of hydroxide base, sulfuric acid or the process of electrodialytic water-splitting, in recovering anhydrous hydrogen fluoride from the ternary liquid system of hydrogen fluoride/fluosilicic acid/water.